Pressure-responsive control device for electrical circuit

ABSTRACT

A control device is provided which is particularly suitable for use as a re-cycle control for air-conditioning and refrigeration systems. The device includes a pair of electrical contacts adapted to be connected in a series with the electrical control circuit of the system and a pair of pressure-responsive diaphragm pumps which open and close the contacts depending upon the relative pressures acting upon the diaphragms. An electrical resistance is connected across the contacts. When the pressure in one of the diaphragms exceeds the pressure in the other diaphragm by a predetermined amount, the contacts will be opened, although the electrical control circuit will be completed through the resistance. However, the resistance will decrease the current flow in the control circuit, and the current flow will not return to its original value until the differential between the pressures acting upon the diaphragms decreases to permit the electrical contacts to close, thereby effectively shorting the resistance out of the circuit.

United States Patent May v [451 May 23, 1972 [72] Inventor: Eugene NoelMay, Aurora, Ill.

-- [73] Assignee: J/B Industries, Inc., Aurora, Ill.

[22] Filed: Sept. 9, 1970 [21] Appl.No.: 70,7l6

Pn'mary Examiner-J. V. Truhe Assistant ExaminerJ. G. SmithAttomey-Dawson, Tilton, Fallon & Lungmus ABSTRACT A control device isprovided which is particularly suitable for use as a re-cycle controlfor air-conditioning and refrigeration systems. The device includes apair of electrical contacts adapted to be connected in a series with theelectrical control circuit of the system and a pair ofpressure-responsive diaphragm pumps which open and close the contactsdepending upon the relative pressures acting upon the diaphragms. Anelectrical resistance is connected across the contacts. When thepressure in one of the diaphragms exceeds the pressure in the otherdiaphragm by a predetermined amount, the contacts will be opened,although the electrical control circuit will be completed through theresistance. However, the resistance will decrease the current flow inthe control circuit, and the current flow will not return to itsoriginal value until the difierential between the pressures acting uponthe diaphragms decreases to permit the electrical contacts to close,thereby effectively shorting the resistance out of the circuit.

9 Claims, 5 Drawing Figures Patented May 23, 1972 EVAPORATOR FIGI DENSERCOMPRESSOR H.P.INLET THERMO- STAT L. P. INLET T w A W /k N w E e W UM E1,7 0 .4! Z w m PRESSURE-RESPONSIVE CONTROL DEVICE FOR ELECTRICALCIRCUIT BACKGROUND This invention relates to a pressure-responsivecontrol device for electrical circuits, and, more particularly, to acontrol device which is particularly suitable for use inair-conditioning and refrigeration systems.

Air-conditioning and refrigeration systems generally include anelectrical compressor for compressing the gaseous refrigerant receivedfrom the evaporator and discharging compressed refrigerant gas to thecondenser. When a selfequalizing type refrigeration system is firststarted up, the refrigerant circuit is substantially in equilibrium, andthe pressure of the refrigerant between the compressor and theevaporator, i.e., the low-pressure side of the refrigerant circult, isapproximately the same as the pressure of the refrigerant between thecompressor and the condenser, i.e., the high-pressure side. When thecompressor is turned on and the compressor begins compressing therefrigerant, the pressure on the discharge or high side of thecompressor increases while the pressure on the suction or low side ofthe compressor decreases. During the operation of the compressor intypical refrigeration systems, the pressure differential between thehigh side and the low side may reach 140 psi, or more.

Refrigeration systems, particularly systems using a permanent splitcapacitor type compressor, generally include a pair of electricalcircuits a line voltage circuit for operating the compressor and acontrol circuit which may include a number of protective devices such asa thermostat, high and low pressure cut-out switches, and the like. Theline voltage circuit is opened and closed by a compressor relay or linevoltage contactor in the control circuit, which is essentially anelectromagnet which moves the contactor to close the line voltagecircuit when the current in the control circuit reaches a certain valueand allows the contactor to move to an open position to open the linevoltage circuit when the control circuit is opened by one of theprotective devices.

If one of these protective devices is actuated, the line voltage circuitwill be opened, and the compressor will shutdown. It is desirable,however, to prevent the line voltage circuit from closing to start upthe compressor again until the pressures on the high side and low sideof the compressor are substantially equalized, even if the irregularitywhich triggered the protective device has passed. The reason for this isthat the compressor generally does not have sufficient starting torqueto pump against a high pressure differential, and the compressor maybecome damaged, fuses for the line voltage circuit may blow, or otherproblems may ensue.

Some protective devices in the control circuit may be provided withtiming devices which are intended to keep the control circuit open for asufficient time to permit the refrigerant system to reach equilibrium.However, it is possible for some unusual transient condition to cause aprotective device to open and then to close upon the passing of thetransient condition before the refrigeration system reaches equilibrium.For example, if cold air is directed against the thermostat for a shortperiod of time, the thermostat will open the control circuit, but whenthe cold air flow is stopped, the thermostat will soon close the controlcircuit. Another problem of increasing frequency is a temporary voltagelag or drop or power failure which may cause the compressor to shut downbut which will not prevent the compressor from attempting to start upagain before pressure equilibrium is reached.

A number of protective devices have been tried in an effort to protectthe compressor from damage due to premature start up or short-cycling,but none of these devices has proved entirely satisfactory. For example,internal or external overloads may be provided so that when thecompressor attempts to start up and pump against a substantial pressuredifferential, the current that the compressor draws will cause theoverload to open the line voltage circuit and shut down the compressor.However, the compressor is still forced to work until the overload opensthe circuit, and operation of the compressor under these conditions foreven a short time may cause damage. Furthermore, the compressor mayattempt to start up several times before sufficient time elapses torestore the refrigerant system substantially to equilibrium.

SUMMARY The inventive control device does not permit the compressor tostart up until the pressure differential between the high and low sideof the compressor reaches a desired minimum. The device includes a pairof electrical contacts which are connected in a series with theelectrical control circuit, and the contacts are directly responsive tothe pressure differential between the high and low side so that thecontacts will not close until the pressure difierential reaches apre-selected value. A resistance is connected in parallel with theelectrical contacts of the device to form a holding circuit for thecontrol circuit when the compressor is operating normally, even thoughthe electrical contacts of the device are open. However, when thecompressor is shut down, the holding circuit resistance (in series withthe electromagnet) will cause the electromagnet to fail to pull in(close) the line voltage contacts to close the line voltage circuituntil the electrical contacts of the device are closed by virtue of thedecrease in the pressure differential, thereby eliminating short-cyclingof the compressor and the possible compressor damage.

DESCRIPTION OF DRAWING The invention will be explained in conjunctionwith an illustrative embodiment shown in the accompanying drawing inwhich:

FIG. 1 is a schematic diagram of a conventional refrigeration system;

FIG. 2 is a sectional view of the control device, the electrical controlcircuit for the compressor being shown schematically;

FIG. 3 is an enlarged fragmentary view, partially broken away, of one ofthe pressure-responsive diaphragm pumps;

FIG. 4 is a sectional view taken along the line of 44 of FIG. 2; and

FIG. 5 is a sectional view taken along the line of S5 of FIG. 2.

DESCRIPTION OF SPECIFIC EMBODIMENT The invention will be described inconjunction with a conventional air-conditioning or refrigeration systemdesignated generally by the numeral 10 in FIG. 1. Such a refrigerationsystem typically includes an electrically powered compressor 11, acondenser 12, and an evaporator 13. The discharge or high-pressure side(I-LP. Side) of the compressor is connected to the condenser by conduit15, and the suction or low-pressure side (LP. Side) of the compressor isconnected to the evaporator by the conduit 16. Conduit 17 connects thecondenser to the evaporator, and a dryer l9 and a metering device 20 maybe interposed in this conduit.

Before the compressor starts up, the refrigerant in the system is undersubstantially the same pressure throughout the system. When thecompressor is started, refrigerant gas is compressed by the compressorand discharged through conduit' 15 to the condenser. The condensedrefrigerant passes through conduit 17, the dryer and metering device tothe evaporator. The refrigerant then returns as a gas through thesuction line 16 to the intake of the compressor. As the compressorcontinues to operate the difference in pressure at the discharge or highside of the compressor and the pressure at the suction or low side, ofthe compressor will increase. The compressor will continue running untilone of the conventional protective devices in the electrical controlcircuit for the compressor causes the line voltage circuit (not shown)to open. After the compressor shuts down, the pressure within thecontinuous refrigerant circuit will eventually become substantiallyequalized throughout the circuit, and the pressure mounted eitherexteriorly or interiorly of the air-conditioner or other refrigerationunit. A pair of diaphragm pumps 24 and 25 are mounted in opposed fashionon the casing, andeach of the diaphragm pumps includes a drive oractuating pin 26 and 27, respectively, which are engageable with a pivotarm 28. The diaphragm pump 24 is connected to the discharge conduit onthe high side of the refrigerant system by the high-pressure inletconduit 29, and the diaphragm pump is connected to the suction conduit16 on the low side of the refrigerant system by the low-pressure inletconduit so that the drive pins 26 and 27 will act upon the pivot arm 28according to the pressure differential between the high and low side ofthe system.

The pivot arm 28, which may be formed of insulating material, ispivotally mounted at one end thereof on the casing 23 on pivot 31, andan electrically conductive contact member 32 is mounted on the other endof the pivot arm. If desired, the contact 32 may be mounted on anelectrically conductive wiring plate 33. An opposing electrical contact34 is carried by an electrically conductive mounting strip 35 whichextends through the casing 23 and is connected to terminal 36. The strip35 and terminal 36 are preferably insulated from the casing, which maybe metal. A second terminal 37 is mounted on the casing and insulatedtherefrom and is electrically connected to the wiring plate 33 by wire38, and an electrical resistance 39 extends between the mounting strip35 and wire 38 in parallel with the contacts 32 and 34.

One end of a biasing spring 40 is secured to the pivot arm 28 adjacentthe contact 32 for maintaining the contacts 32 and 34 in electricalengagement when there is'little or no pressure differential between thehigh and low sides of the refrigerant system. The other end of thespring is mounted on the casing 23 by means of adjusting screw 41, whichmay be used to adjust the spring tension on the pivot arm as desired.

Referring to H6. 3, the diaphragm pump 25 may be conventional andincludes a housing 43 having an interior chamber 44. A flexiblediaphragm 45 is mounted within the housing and divides the chamber 44into upper and lower portions 44a and 44b, respectively. The drive pin27 extends slidably through a gasket or grommet 46 secured to thehousing 43 and the inner end of the drive pin is secured to thediaphragm 45 for movement therewith by attaching screw 47.

. The lower chamber 44b communicates with the low pressure side of therefrigeration system through the low pressure inlet conduit 30, and thediaphragm 45 is responsive to the pressure of the refrigerant in thesuction conduit 16, the drive pin 27 being urged upwardly as thepressure of the refrigerant on the low side increases and beingretracted downwardly as the pressure on the low side decreases.

The diaphragm pump 24 is similarly constructed, and the drive pin 26 ismoved downwardly as the pressure of the refrigerant on the high sideincreases and moved upwardly as the pressure on the high side decreases.

The control device 21 is shown in FIG. 2 in its equilibrium conditionwhen the pressures on the high side and low side of the refrigerantsystem are substantially equal. In this condition, the electricalcontacts 32 and 34 are maintained in electrical contact by the spring40. The spring need provide only enough tension to hold these contactsclosed when the pressure differential between the high and the low sideis substantially zero. However, if desired, the tension of the springcan be adjusted to close the electrical contacts when the pressuredifferential reaches some other predetermined value. For example, aparticular compressor might be able to start up safely against apressure differential of, say, 15 psi, and the spring tension may beadjusted by screw 41 to close the contacts when the pressuredifferential is reduced to this level. If desired, the mounting strip 35for the contact 34 my be formed of flexible, resilient material such asspring metal which will permit the contact 34 to move upwardly slightlyunder the force of the spring but which will return the contact to itsoriginal position when the contacts are opened.

The electrical terminals 36 and 37 of the control device are connectedin series to the control circuit 22 for control of the line voltage orcompressor circuit. The control circuit 22 may be conventional, and thistype of circuit generally includes a step-down transformer 48 to providea voltage at the secondary windings 49 below line voltage, andconventional protective devices such as a thermostat 50 and low-pressureand highpressure cut-out switches 51 and 52, respectively. A linevoltage contactor or compressor relay electromagnet 53 is also connectedin a series in the electrical control circuit. The line voltagecontactor serves to open and close the line voltage circuit (not shown)to operate the compressor when the refrigeration unit is turned on andoff and to open the line voltage circuit when one of the protectivedevices is actuated to open the control circuit 22.

When the compressor is started, the pressure differential between thehigh and low side of the compressor will immediately begin to increase,and drive pin 26 will urge the pivot arm 28 and the drive pin 27downwardly, breaking the electrical contact between the contacts 32 and34. However, even though the contacts are separated, the electricalcontrol circuit 22 will not be open since the circuit will be completedthrough the resistor 39 which is connected across the ter-,.

minals 36 and 37. The resistor is chosen to permit sufficient current toflow through the control circuit to hold the compressor relayelectromagnet 53 in the closed" position to maintain a line voltagecircuit which operates the compressor. The resistance is sufficient,however, to prevent the flow of sufficient current to activate theelectromagnetic relay from the open" position to the closed position.

If one of the protective devices opens the control circuit 22 after thecontacts 32 and 34 have become separated, the electromagnet of thecompressor relay will immediately deenergize (open), thereby opening theline voltage circuit and shutting down the compressor.

At times the protective. device that has opened the control circuit willclose before the pressure in the refrigerant system has equalized. Forexample, the thermostat 50 may open due to an unusual transientcondition in which cold air is blown at the thermostat or in which thethermostat control is turned upward and then turned to a lower position.The thermostat will open the control circuit for only a short time andthe thermostat will close before the refrigerant pressure has beenequalized. When the protective device closes, the control circuit willbe completed through the resistor 39, and current will begin to flowthrough to the compressor relay electromagnet. However, because of theresistor value the current flow will not be sufficient to pull in theelectromagnet of the compressor relay from the open" position to theclosed position to close the line voltage circuit. Sufficient current toactivate the compressor relay electromagnet will not flow until thepivot arm 28 has been raised to close the contacts 32 and 34 so that thecurrent can bypass the resistor 39. As soon as the contacts 32 and 34close, the compressor relay electromagnet will be fully activated andthe line voltage circuit contacts will be closed to start up thecompressor. Thus, short-cycling of the compressor is prevented, and thecompressor will not start until the pressure on both the high side andthe low side has been substantially equalized.

A typical control circuit will operate at 24 volts by virtue of thestep-down transfonner 48. For such a 24-volt control circuit, a 0.5-ampscompressor relay is conventional, i.e., the compressor relay will notswitch from the open position to the closed" position until 0.5 ampsflows through the electromagnet. The resistor 39 is chosen to permit thecurrent to flow through the control circuit at a value slightly lessthan the actuating current required for the relay electromagnet. For oneparticular 24-volt control circuit which included an RBM line voltagecontactor No. 95693, the actuating current was 0.5 amps, and I obtainedexcellent results with a 75 ohm/l0 watt resistor. At 24 voltsapproximately 0.32 amps will flow through this resistor, and thecompressor relay will not pull in. However, when the contacts 32 and 34close, the resistor will be bypassed and sufficient current will flowthrough the control circuit to actuate the relay. It will be understood,however, that many values of resistance can be used for a particularcircuit. The important thing is for the resistor to prevent sufficientcurrent flow to actuate the compressor relay until the contacts 32 and34 are closed but to permit sufficient current flow to hold thecompressor relay in the on or closed position until one of theprotective devices opens the control circuit.

Some control circuits operate at l volts, and others operate at 220volts. For these circuits different values of resistance will be useddepending upon the actuating current for the particular compressor relayin the control circuit.

If the control device 21 is provided as part of the original equipmenton the refrigeration unit, the manufacturer can readily select theappropriate resistor 39 for the control circuit. However, the controldevice can also be installed by a service man separate from the originalequipment. The service man can first determine the load circuit value ofthe control circuit by using an amprobe multiplier bar or equivalentdevice, and a resistor is then selected which will reduce the currentflow slightly below the load circuit. The electrical terminals 36 and 37of the device can readily be attached in series to the control circuit,and the high and low pressure inlets 29 and 30 can be connected to thedischarge line and suction line 16, respectively, by conventionalrefrigerant linetapping valves, or the like.

In the particular embodiment illustrated, the tension of the spring 40was adjusted to provide merely a slight force to hold the contact 32against the contact 34 when the pressures in the-diaphragm pumps 24 and25 were approximately equal. However, certain compressors may be able tostart up against a slight pressure differential without damage, and insuch a case the tension of the spring 40 can be increased so that thecontacts 32 and 34 will close before the pressure in the refrigerantsystem has reached equilibrium.

In the particular embodiment illustrated, a diaphragm pump was providedfor both the high-pressure side and the low-pressure side. However, ifthe refrigeration unit will normally operate at a substantially constantambient temperature, the control device could be provided with adiaphragm pump for only the high-pressure side, and the tension of thespring 40 could be adjusted to provide sufficient force to return thedrive pin 26 to is neutral position in which the contacts close when thepressure acting on the high pressure diaphragm returns to theequilibrium pressure. For example, a particular refrigeration system mayhave an equilibrium pressure of 75 psi when the ambient temperature is80 F. The spring 40 can then be adjusted to close the contacts 32 and 34when only about 75 psi pressure acts against the high-pressurediaphragm. However, since the ambient temperature of the refrigerationunit generally varies, the equilibrium pressure will also vary, and itis desirable to provide a diaphragm for both the high-pressure side andthe low-pressure side to ensure closing of the contacts when therefrigerant system reaches equilibrium.

However, a control device provided with a diaphragm pump for only thehigh-pressure side could provide an ambient temperature lock-outcontrol. For example, if the diaphragm pump 25 was eliminated and thespring force on the pivot arm were reversed so that the spring urged thecontacts apart, the spring force could be adjusted so that the contactswould not close even after the refrigerant system reached equilibriumunless equilibrium pressure reached a certain minimum value.

While in the foregoing specification, a detailed description of specificembodiments were set forth for the purpose of illustration, it is to beunderstood that many of the details herein given may be variedconsiderably by those skilled in the art without departing from thespirit and scope of my invention.

I claim:

1. A pressure-responsive control device for an electrical circuitcomprising a frame, a first, electrical contact on the frame, contactmounting means movably mounted on the frame for movement toward and awayfrom the first electrical contact, a second electrical contact carriedby the mounting means, pressure-responsive means including a membermovable under the influence of pressure which acts upon thepressure-responsive means, the movable member being engageable with saidcontact mounting means to move the second contact out of electricalengagement with the first contact, mwans for urging the second contactinto electrical engagement with the first contact, and an electricalresistance electrically connected across the first and second contacts.

2. The device of claim 1 in which said pressure-responsive meansincludes a pressure-responsive flexible diaphragm and said movablemember is attached to the diaphragm for movement therewith in responseto pressure acting on the diaphragm.

3. The device of claim 1 in which said urging means include a secondpressure-responsive means including a member movable under the influenceof pressure acting on the second pressure-responsive means, the movablemember of the second pressure-responsive means being engageable with thecontact mounting means for urging the second contact into electricalengagement with the first contact.

4. The device of claim 1 in which said urging means includes springmeans for urging the second contact into electrical engagement with thefirst contact.

5. The device of claim 1 in which said mounting means comprises a pivotarm for pivoting said one contact toward and away from'the othercontact, the movable member of the pressure-responsive means beingengageable with said pivot arm.

6. A control apparatus for a refrigeration system having an electricallypowered compressor, low pressure conduit means for supplying refrigerantto the compressor, high pressure conduit means for conductingrefrigerant away from the compressor, and an electrical power circuitfor supplying electric current to the compressor, the control apparatuscomprising an electrical control circuit having a source of electriccurrent, first and second electrical contacts, electromagnetic relaymeans connected in series with the electrical contacts, the relay meansincluding contact means movable between a first position in which theelectrical power circuit is open and a second position in which theelectrical power circuit is closed by the contact means, a resistorconnected across the electrical contacts, frame means, one of theelectrical contacts being mounted on the frame means, contact mountingmeans movably mounted on the frame means for supporting the otherelectrical contact, first and second pressure-responsive means on theframe for connection to, respectively, the high pressure conduit meansand the low pressure conduit means of the refrigeration system, thefirst pressure-responsive means being engageable with the contactmounting means for urging said other electrical contact away from saidone electrical contact under the influence of pressure in the highpressure conduit means, the second pressure-responsive means beingengageable with the contact mounting means for urging said otherelectrical contact toward said one electrical contact under theinfluence of pressure in the low pressure conduit means, current flowthrough the electrical control circuit when the electrical contacts arein electrical engagement being sufficient toenergize the electromagneticrelay means to move the contact means thereof from the first position tothe second position, the resistance of the resistor being such thatsufficient current flows through the electrical control circuit tomaintain the contact means of the relay means in the second positionwhen the electrical contacts are separated but that insufficient currentflows through the electrical control circuit to move the contact meansof the relay means from the first to the second position when theelectrical contacts are separated, the contact means of the relay meansmoving from the second to the first position when current flow throughthe control circuit ceases.

7. The apparatus of claim 6 including a spring on the frame connected tothe contact mounting means for urging the other electrical contacttoward said one electrical contact.

8; A control apparatus for a refrigeration system comprising a frame, afirst electrical contact on the frame, a mounting member movably mountedon the frame, a second electrical contact mounted on the mounting memberfor movement toward and away from the first electrical contact to bringthe electrical contacts into and out of electrical engagement, first andsecond pressure-responsive means on the frame, the firstpressure-responsive means being engageable with the mounting member forurging said second electrical contact away from the first electricalcontact under the influence of pressure in the first pressure-responsivemeans, the second pressure-responsive means being'engageable with saidmounting member for urging said second electrical contact towards thefirst electrical contact under the influence of pressure in the secondpressure-responsive means, the first and second electrical contactsbeing electrically connected in series with an electromagnetic relaymeans and a source of electric current to provide an electrical controlcircuit, a resistor connected across the electrical contacts to providea holding circuit when the electrical contacts are separated, theelectromagnetic relay means including contact means movable between afirst position and a second position, the contact means of the relaymeans being in the first position when no current flows through thecontrol circuit or holding circuit, the relay means being energizeableto move the contact means thereof from the first position to the secondposition when the electrical contacts are in electrical engagement, theresistance of the resistor being such that sufficient current flowsthrough the holding circuit to maintain the contact means of the relaymeans in the second position when the electrical contacts move out ofelectrical engagement but that insufficient current flows through theholding circuit to move the contact means from the first to the secondposition when the electrical contacts are out of electrical engagement,the contact means of the relay means returning from the second to thefirst position when current flow through the control circuit or holdingcircuit ceases.

9. The apparatus of claim 8 including a spring on the frame

1. A pressure-responsive control device for an electrical circuitcomprising a frame, a first, electrical contact on the frame, contactmounting means movably mounted on the frame for movement toward and awayfrom the first electrical contact, a second electrical contact carriedby the mounting means, pressure-responsive means including a membermovable under the influence of pressure which acts upon thepressure-responsive means, the movable member being engageable with saidcontact mounting means to move the second contact out of electricalengagement with the first contact, mwans for urging the second contactinto electrical engagement with the first contact, and an electricalresistance electrically connected across the first and second contacts.2. The device of claim 1 in which said pressure-responsive meansincludes a pressure-responsive flexible diaphragm and said movablemember is attached to the diaphragm for movement therewith in responseto pressure acting on the diaphragm.
 3. The device of claim 1 in whichsaid urging means include a second pressure-responsive means including amember movable under the influence of prEssure acting on the secondpressure-responsive means, the movable member of the secondpressure-responsive means being engageable with the contact mountingmeans for urging the second contact into electrical engagement with thefirst contact.
 4. The device of claim 1 in which said urging meansincludes spring means for urging the second contact into electricalengagement with the first contact.
 5. The device of claim 1 in whichsaid mounting means comprises a pivot arm for pivoting said one contacttoward and away from the other contact, the movable member of thepressure-responsive means being engageable with said pivot arm.
 6. Acontrol apparatus for a refrigeration system having an electricallypowered compressor, low pressure conduit means for supplying refrigerantto the compressor, high pressure conduit means for conductingrefrigerant away from the compressor, and an electrical power circuitfor supplying electric current to the compressor, the control apparatuscomprising an electrical control circuit having a source of electriccurrent, first and second electrical contacts, electromagnetic relaymeans connected in series with the electrical contacts, the relay meansincluding contact means movable between a first position in which theelectrical power circuit is open and a second position in which theelectrical power circuit is closed by the contact means, a resistorconnected across the electrical contacts, frame means, one of theelectrical contacts being mounted on the frame means, contact mountingmeans movably mounted on the frame means for supporting the otherelectrical contact, first and second pressure-responsive means on theframe for connection to, respectively, the high pressure conduit meansand the low pressure conduit means of the refrigeration system, thefirst pressure-responsive means being engageable with the contactmounting means for urging said other electrical contact away from saidone electrical contact under the influence of pressure in the highpressure conduit means, the second pressure-responsive means beingengageable with the contact mounting means for urging said otherelectrical contact toward said one electrical contact under theinfluence of pressure in the low pressure conduit means, current flowthrough the electrical control circuit when the electrical contacts arein electrical engagement being sufficient to energize theelectromagnetic relay means to move the contact means thereof from thefirst position to the second position, the resistance of the resistorbeing such that sufficient current flows through the electrical controlcircuit to maintain the contact means of the relay means in the secondposition when the electrical contacts are separated but thatinsufficient current flows through the electrical control circuit tomove the contact means of the relay means from the first to the secondposition when the electrical contacts are separated, the contact meansof the relay means moving from the second to the first position whencurrent flow through the control circuit ceases.
 7. The apparatus ofclaim 6 including a spring on the frame connected to the contactmounting means for urging the other electrical contact toward said oneelectrical contact.
 8. A control apparatus for a refrigeration systemcomprising a frame, a first electrical contact on the frame, a mountingmember movably mounted on the frame, a second electrical contact mountedon the mounting member for movement toward and away from the firstelectrical contact to bring the electrical contacts into and out ofelectrical engagement, first and second pressure-responsive means on theframe, the first pressure-responsive means being engageable with themounting member for urging said second electrical contact away from thefirst electrical contact under the influence of pressure in the firstpressure-responsive means, the second pressure-responsive means beingengageable with said mounting member for urging said second electricalcontact tOwards the first electrical contact under the influence ofpressure in the second pressure-responsive means, the first and secondelectrical contacts being electrically connected in series with anelectromagnetic relay means and a source of electric current to providean electrical control circuit, a resistor connected across theelectrical contacts to provide a holding circuit when the electricalcontacts are separated, the electromagnetic relay means includingcontact means movable between a first position and a second position,the contact means of the relay means being in the first position when nocurrent flows through the control circuit or holding circuit, the relaymeans being energizeable to move the contact means thereof from thefirst position to the second position when the electrical contacts arein electrical engagement, the resistance of the resistor being such thatsufficient current flows through the holding circuit to maintain thecontact means of the relay means in the second position when theelectrical contacts move out of electrical engagement but thatinsufficient current flows through the holding circuit to move thecontact means from the first to the second position when the electricalcontacts are out of electrical engagement, the contact means of therelay means returning from the second to the first position when currentflow through the control circuit or holding circuit ceases.
 9. Theapparatus of claim 8 including a spring on the frame connected to thecontact mounting means for urging the other electrical contact towardsaid one electrical contact.